Field of the Invention
The present invention relates to a method for operating an x-ray system for examining an object. Moreover, the invention relates to such an x-ray system.
Published, non-prosecuted German patent applications DE 10 2013 226 242 A1 and DE 10 2006 044 783 A1 disclose methods for operating an x-ray system, in which the actual recording region of the medical x-ray recording, e.g. the start and end position in the case of CT imaging, is adjusted using an overview record. There, this recording region is adjusted by way of example by gestures of the user using a touch screen and/or cameras, or else by manual or automatic placement of a marking characterizing the recording region in the overview record. The actual x-ray recording is carried out following the adjustment of the recording region.
When examining an object with the aid of an x-ray system by generating fluoroscopic images, it is necessary, for various reasons, to distinguish between relevant and non-relevant image contents. Such a distinction is important, firstly, for adapting the radiation dose used to pass radiation through the object. Secondly, such a distinction is important to the image processing (image post-processing) of the fluoroscopic images.
The signal-to-noise ratio, with which an object situated in the beam path is imaged on a detector, is proportional to the transmitted radiation dose. The dose measured at the image receiver (x-ray detector) is therefore an indirect measure for the image quality and it is often predetermined as a setpoint variable for the dose regulation. In the case of systems without a detector-side ionization chamber, the radiation dose is usually calculated directly from the recorded image data (grayscale values). This is possible as the unprocessed image data are directly proportional to the absorbed radiation dose in digital x-ray systems.
Since the image data often also include regions without clinical relevance, e.g. direct radiation or metal objects, in addition to anatomical information, a decision needs to be made as to which image regions should be used for calculating the radiation dose. Advantageously, only those image regions which have relevant image contents are used for calculating the dose and, consequently, for regulating the radiation dose. The dose regulation is then based upon establishing the grayscale value in a set ROI (region of interest). Here, the intensity of the x-ray radiation is controlled in such a way that a constant mean value emerges.
Independently of a dose calculation, the image data of the fluoroscopic images must also be prepared in such a way that relevant information (e.g. clinically relevant information in the case of medical x-ray systems) is depicted in an ideal manner to the user. Here, a common problem is that the anatomical, potentially clinically relevant data also cover a large grayscale value range, which cannot be presented in an ideal manner to the user as a whole. Therefore, there must also be a selection of relevant image regions in the fluoroscopic image, even in the case of the image processing, which image regions are subsequently prepared for the user in an ideal manner.
Modern digital x-ray systems use one or more of the following methods for automatic image evaluation to distinguish between relevant image contents (e.g. clinically relevant anatomy) and other image contents.
In a first method, use is made of fixed measurement fields. Here, static image regions are set in relation to the image matrix, which image regions are evaluated for determining the radiation dose. The user can ensure correct dose regulation and image processing by virtue of setting the beam geometry in such a way that the part of interest of the object is imaged on this image region. The main disadvantage of this method consists of the fact that the user is restricted in terms of his workflow, particularly in respect of positioning the object and x-ray system.
In addition to this position-dependent object evaluation, a histogram regulation is applied as a second method. Here, relevant and non-relevant grayscale value regions are established dynamically with the aid of predefined rules using a histogram of the recorded image data. The goal is to ensure a position-independent object evaluation. The histogram regulation cannot ensure a correct evaluation of the image in specific clinical applications, e.g. lateral cervical spine or objects, e.g. very large metal implants. Then, there is a risk of the wrong image contents being considered relevant and therefore the dose regulation and/or image processing not being carried out correctly.
Manual regulation can be provided as a third method. Here, operating elements are offered to the user, with the aid of which the user can directly influence the parameters (current, voltage) of the x-ray apparatus in order to correct overexposure or underexposure on the part of the automatic image evaluation. Disadvantages of this method are the susceptibility to errors in the case of an incorrect interpretation of the user, the poor operability and the often non-ideal coupling to the image processing.
What should be noted overall is that, on account of the large scope of possible clinical applications and objects, it is not possible to exclude the case that an automated image evaluation with the above-described methods, including a possible manual correction, fails. This may occur both when establishing the relevant image contents for the dose regulation and during image processing. The possible consequences of an incorrect evaluation are, inter alia, an increased dose exposure for the object and operating staff and poor image quality due to underexposure or overexposure of the relevant structures or due to non-ideal preparation of the image data by the image processing.